Loosely speaking, in the downstream case a player at node \(i\)  acts as a potential donor towards player \(j\)  in the interaction network, and player \(j\)  may later choose to imitate the behavior of \(i\)  and act in the same way towards a third party. In the upstream case, by contrast, a player at node \(i\)  acts as a potential donor to player \(j\) , and a third party may later choose to imitate the behavior of \(i\)  and take it against \(i\) . When all edges are bidirected, the evolutionary process of interaction and strategic imitation are the same in both upstream and downstream cases, and they coincide with classical models in structured populations [1]}, [2]}, [3]}.
